The present invention relates to a method of making a printed circuit board, and more particularly to a method of making a printed circuit board which enables the formation of a fine wiring pattern on the board""s substrate.
FIGS. 1(A)-1(G) are diagrams showing a conventional method for fabricating a printed circuit board. FIG. 1(A) will hereinafter be referred to as xe2x80x9cstep (A)xe2x80x9d, FIG. 1(B) as xe2x80x9cstep (B)xe2x80x9d, etc. in the following description prior to the description of FIGS. 2 and 3. First, in step (A), a substrate 10 having a metal layer 12 (e.g., copper foil) on each of the surfaces thereof is prepared, and in step (B), a through hole 14 is formed in the substrate 10, including through the copper foil. Then, in step (C), plating is performed to deposit a metal film 16 (e.g., copper) on the surfaces of the inner wall of the through hole 14 and the copper foil 12. In step (D), the through hole 14 is filled with an insulating material 18, such as a resin. Then, in step (E), plating is again performed to deposit a film 20 (e.g., copper) on the surfaces of the substrate 10, including the top and bottom surfaces of the resin in through hole 14. In step (F), the copper films 12, 16 and 20 on the substrate 10 are patterned or circuitized to form a wiring pattern 23. Following this, in step (G), a resin layer 24 is applied on the surfaces of the substrate 10, and is patterned to form a via 26 as needed. Again, copper plating is performed to deposit a copper film on the resin layers 24 and is patterned to form a wiring pattern 28. By repeating the application of the resin layer, and the plating and patterning of the copper film as needed, a wiring structure having multiple thin wiring layers can be provided.
According to the above conventional method, since the total thickness of the external copper material (layer) in step (E) is the total thickness of the individual copper films 12, 16 and 20, this copper layer cannot be finely patterned. If one tries to form a fine circuit pattern with small line-to-line spacings (gaps), the copper layer may be incompletely etched, as is indicated by a portion 22 in Step F. As a result, a short-circuit may occur in the wiring pattern. Further, in step (G), when the resin layer 24 is applied, air may be trapped in a small gap between the wiring conductors, such that an air bubble 30 may be formed. Such an air bubble may impair continuity of the resin layer 24 and its insulating properties. It has been found that such air entrapment is more likely to occur when the total thickness of the copper layer is greater than 25 xcexcm.
It is, therefore, one object of the present invention to provide a method of making a printed circuit board in which a fine wiring pattern can be formed.
It is another object of the present invention to provide such a method of making a printed circuit board which can be performed in a facile manner.
According to one aspect of the invention, there is provided a method of making a printed circuit board comprising the steps of preparing a substrate having at least two opposing surfaces, providing an initial copper layer on each of the opposing surfaces, forming through holes, each having inner walls, in the substrate, treating the substrate with a catalyst to form a catalyzed layer on the substrate, including the inner walls of the through holes, and then plating the treated surfaces of the substrate with copper. The through holes are then filled with an insulating material, followed by thinning the copper layer on the at least one surface so that no part of the catalyst layer is exposed on the surface, removing any insulating material projecting from the surface of the thinned copper layer to thereby level the at least one surface, plating the at least one surface of the substrate and the insulating material with copper, and patterning the copper layer on the at least one surface to form a first wiring pattern.